This was my problem as will. I bought a beefed up bracket from another Mini supplier that bolts in place. I thought of welding mine back on but figured the SS bracket would be a much more substantial solution. I fabbed up a bracket for the backside with captured nuts to make installing easier. Also, on the one I bought, you can use 4 bolts and I added washers. Maybe mine were overkill as I was not able to use the third bolt that is directly under where the dog bone/engine steady bolts. They interfered. I sealed that hole and added a little sealant to the bracket so it was water tight.

I drilled through and bolted. I will have to look later today to see which size and which side the nuts and washers went on. Since the engine is pulling/pushing on the firewall at this point, self tapping screws would not be enough. I do recall I was able, with determination, to do it alone. I don't remember if I had to remove the heater hoses to gain hand access up under the parcel shelf and into the cross-member.

Update: 2 @ 1/4" bolts from the engine side into a 1"x1" steel angle and through the firewall, with the nuts and washers on the inside where they wouldn't corrode. I tried to get pictures under the dash, but the carpet and heater air intake tube are in the way.
Looking from the engine bay side, I have no idea how I managed to drill the holes through the firewall though!

did you access the inside of the cross member for a nut/bolt combination, or did you use self-tapping screws or something like that?  

Thanks, I'll check out the floor too.  My plan is same as yours but I'm adding some body panel sealant between the two "plates" before I bolt them together.  

Good to know! Mine had the same problem.

If you scroll back to my post of July 21, 2022, you can see and read about my solution. Not having a welder, I made a new lower bracket out of steel angle and bolted it on through the cross-member.

You might want to check the right side floor where the subframe tails are bolted on. With an engine flopping around it puts leverage on the subframe and the tail causes the floor panel to oilcan and eventually crack from metal fatigue. That's where my telltale snap or clunk sound was coming from. Fixing the engine steady solved the floor noise issue. Hopefully your noise was just the wishbone tapping against the firewall cross-member.

Found the problem.  Bulkhead bracket for the engine steady is broken off at the weld.  the part where the engine steady bolt and the clutch line pass through is separated from the half that is welded to the bulkhead.  Hoping I can weld a new bracket onto the existing piece.  

Maybe you found ML in your boot ..?/ he has been missing for years.just . like Jimmy Hoffa.. 

When you disconnected the upper engine steady, did you remove the bolts from both ends and inspect the  brackets at the firewall? That end can be deceiving.

Engine torque and arm muscle torque are quite different. Assuming you don't have gorilla hands, your hands can not rock the motor as nearly as much as the engine can.

Try this: Put the transmission in 1st gear and release the parking brake. With the bonnet open and the engine off, grasp the slam panel and push/pull the car back and forth as hard as you can. You will get a little movement of the car each way as you take up the gear train slack. When all the slack is taken up the small but significant momentum of the car will be enough to almost turn the engine (it will easiest in 4th gear). You want 1st gear so the engine has the greatest mechanical advantage against the mass of the car. You will see a lot more rocking of the engine than you could do by hand and perhaps reproduce your clunk/ thunk/ thud/ crack.

So, I disconnected the upper mount and found no issues.  I shook the engine back and forth with the upper mount disconnected and could not duplicate the sound nor could I "feel" anything making contact.  I thought for sure that would produce something.

Always hard to answer these because one mans thunk is another mans thud so the more the merrier, especially if they are just things to check.

 

On later cars, there a few places to check...I know because I have suffered from ALL of them (which I figured out why eventually. ha).

Certainly check the upper engine steady carefully. Mine sheared so cleanly that you would never see it unless you really rocked the motor by hand why shining a bright line down there.

It is also worth checking the lower engine steady (if fitted) as they can crack at the subframe. 

If a later car, the subframes were mounted to the heel board and its not ideal. They can often crack at the mounting points. Mine sounded like a clicking every time I was on and off the gas eventually. Pull the carpet back under the pedals and across and check them. Usually captured bolts, connected by a strip a steel.

 

Again, its always a good time to check everything while you are already poking around.

if it's not on pot joints could be a U joint hitting the exhaust downpipe.

thanks for the input Dan, Willie_B, and Rosebud.  I'll be going through her engine compartment this weekend to hopefully find the problem.  

I think it could be an upper engine steady. Later Minis (A+ engines?) have a lower stock steady to the transmission which does stiffen things. If the main motor mounts are good and you have a HIF44 installed, it would be very close to the firewall and the carb would hit the firewall or maybe even the brake and clutch lines (left hand drive) the carb. The attached picture shows a HIF44 with alloy intake on a 1275 A+.

When checking the upper engine steady, Disassemble it and inspect carefully the firewall mounts. On my car, the lower mount was not properly welded in the factory to the cross-member, had pulled loose sometime inn its history and had been brazed back on, only to come loose again. (Brazing doesn't stick to paint, apparently.)

I discovered this when the engine steady bolt fatigued a hole in the upper mounting plate and the engine steady became partially useless. (I discovered this when trying to solve a creaking/cracking noise in the right hand footwell - occurring during acceleration/decelleration. The engine was flexing the subframe resulting in a crack being torn in the footwell floorboard where the sub-frame bolts on.

So, in the photo you can see the aluminum checker plate I made to replace the upper support and the original lower support with the clutch line still connected to it - I did not want to have to deal with clutch hydraulics at the time. The lower bracket was replaced with s short piece of steel angle bolted through the cross-member. (That was fun bolting up alone!)

I have heard of others finding the lower bracket has come loose.

At the engine end of the steady, check the bolts and brackets are in good condition, as well as both rubber bushings at each end.

Engine steady bushing bad or gone, steering rack end bushing bad, something loose and moving in the boot or inside the car?

Seems to me that if it were a motor mount/engine steady issue, you would "feel" the clunk as well, particularly in the steering wheel.

Hard on the gas and quick shifting this morning while getting up to speed on the highway....now I get a "thunking" sound when hitting the gas.  Motor mount broken or bushing maybe?  1275 w/4 speed.